Aqueous synthesis of ionically conductive compositions of matter

ABSTRACT

An aqueous process for the preparation of a solid ionically conductive composition of matter having the formula MAg4I5 in which M is Rb, K, NH4, Cs, or a combination of these, Cs being present only as a minor constituent of M, comprising preparing a substantially saturated solution of MI in water, intimately admixing four molar equivalents of AgI with the MI solution to effect reaction between the MI and AgI, and removing the water to recover MAg4I5 as the synthesized ionically conductive composition.

United States atent 1 Johnston 51 March 6, 1973 [54] AQUEOUS SYNTHESISOF IONICALLY CONDUCTIVE COMPOSITIONS OF MATTER [75] Inventor: William V.Johnston, Camarillo,

[21] Appl. No.: 96,229

[52] US. Cl ..423/463, 252/518 [51] Int. Cl ..H0lb 1/06, C0lb 9/00, C0lgl/06,

C0lg 3/04, C0lg 7/00, C0lg 9/04 [58] Field of Search ..252/5l8; 23/87,89, 367

[56] References Cited UNITED STATES PATENTS 3,519,404 7/1970 Argue et al..252/518 2,944,975 7/1960 Folberth ..23/204 OTHER PUBLICATIONS C.Brink, et al., Acta Crystallegraphica July l952,

Vol. 5, PP. 433-436 Jacobson: Encycopedia of Chemical Reactions, Vol.VI, i956, p. 162.

Primary Examiner-George F. Lesmes Assistant ExaminerJ. P. BrammerAttorney-L. Lee Humphries, Thomas S. MacDonald and Henry Kolin 5 7ABSTRACT An aqueous process for the preparation of a solid ionicallyconductive composition of matter having the formula MAg l in which M isRb, K, Nl-l Cs, or a combination of these, Cs being present only as aminor constituent of M, comprising preparing a substantially saturatedsolution of MI in water, intimately admixing four molar equivalents ofAg! with the MI solution to effect reaction between the MI and Agl, andremoving the water to recover MAg l as the synthesized ionicallyconductive composition.

9 Claims, No Drawings AQUEOUS SYNTHESIS OF IONICALLY CONDUCTIVECOMPOSITIONS OF MATTER BACKGROUND OF THE INVENTION This inventionrelates to a process for the preparation of ionically conductivecompositions of matter having unusually high ionic conductivity. Itparticularly relates to an aqueous process for the preparation ofcompounds having the formula MAg l in which M IS Rb, NR K, Cs, or acombination of these, Cs being present only as a minor constituent of M.Such compositions are particularly useful as electrolytes in solid stateelectric cells. Such compositions are shown and described in U. S. Pat.No. 3,519,404; electric cells containing these electrolyte materials aredescribed in U. S. Pat. No. 3,443,997.

Compositions of the foregoing type, MAg I as well as certain organicammonium silver iodide compositions have recently been shown to haveionic conductivity in the solid state over a wide range of temperatures.The known methods of preparing the ionically conductive alkali metalsilver iodides involve mixing appropriate amounts of the solid reactantsMI and Agl in the desired 1:4 molar ratios, and either heating themixture until molten, followed by a holding period at a lowertemperature to form the desired compound (melt-anneal technique), orreacting the solid reactants in a time-consuming solid state reaction at200C or higher to form the compound. In an alternative method ofpreparation of MAg I the constituents are dissolved in acetone andprecipitated by evaporation. However, this method results in a mixtureof the desired compound and another compound, generally a highresistivity compound having the formula M,AgI (2 MI:AgI) rather than thepure ionically conductive composition. For preparing the organic silverammonium iodide conductive compositions, a melt-anneal technique isgenerally used. A paste preparation technique has also been used inwhich a slurry or paste is prepared of the silver iodide and the organicammonium iodide, followed by a multiple annealing technique. Synthesisin an aqueous medium in which the silver iodide and organic ammoniumiodide reactants and the formed product exhibit only a limitedsolubility has also been suggested as feasible.

The present invention provides a simple and convenient method for thepreparation of solid ionically conductive alkali metal silver iodidesfrom selected solutions. The process of this invention is advantageouscompared with the methods known to the prior art in that it is moreeasily adapted to large-scale production, does not require the use of ahigh temperature, and may be used to produce large crystals or thinfilms of the desired compound.

The compositions prepared by the process of this invention have a highionic conductivity and may be used in a variety of electrochemicaldevices. The process is of particular utility for preparing compositionsfor use in thin-film devices.

SUMMARY OF THE INVENTION The solid ionically conductive compositionswhich may be prepared by the process of this invention have the generalformula MAgJ, in which M is Rb, K, NH Cs, or combinations thereof, Csbeing present only as a minor constituent of M.

According to the process of this invention, an aqueous essentiallysaturated solution of MI is prepared. This is prepared conveniently andpreferably at room temperature, although any temperature up to the boil-5 ing point of the saturated aqueous alkali metal iodide solution may beused. Four molar equivalents of silver iodide are then intimatelyadmixed with this saturated solution to effect the desired reaction. Thewater is then removed so as to recover the formed MAg l ionicallyconductive composition therefrom.

C. Brink et al. in Acta Cryst. 5, 433-36, (1952) have described thepreparation of the nonconductive compositions K Agl Rb Agl and (NII,,)AgI by saturating a warm concentrated solution of the alkali metaliodide with silver iodide. They found that one mole of KI, Rbl, or NI-II would react with only half a mole of Agl, thereby forming thenonconductive compounds shown.

It has now been discovered that it is feasible to react four molarequivalents of Agl for each molar equivalent of MI present in asaturated aqueous solution of MI so that the reaction that occursbetween the silver iodide and the alkali metal iodide results information of the desired ionically conductive alkali metal tetrasilverpentaiodide composition as the final product obtained, substantiallyfree of the Brink et al compound. Continuous stirring is preferablyemployed as the entire calculated amount of silver iodide is graduallyadded as a fine powder to the saturated aqueous alkali metal iodidesolution. Initially, the resulting mixture rapidly thickens, suggestingpossible formation of the Brink et al compound as an intermediate. Then,with continued addition of Agl, the mixture begins to thin out to a morefluid paste, suggesting that formation of the desired compound hasoccurred. While not essential, it is preferable to facilitate the mixingand reaction by warming the solution. Further, by the addition of aslight excess of water or by raising the temperature, a clearamber-colored solution is obtained. After reaction has occurred, asindicated by formation of the paste or of the clear solution, the waterpresent is removed by evaporation at room temperature or by drying in anoven. The process results in the formation of MAg l and the relativeabsence of the nonconductive M Agl DESCRIPTION OF THE PREFERREDEMBODIMENTS In the practice of the process of the present invention inits preferred aspects, a minimum amount of water is utilized fordissolving the appropriate amount of the alkali metal iodide,particularly rubidium iodide. While a near-saturated solution may beused initially, depending in part on the reaction temperature, it ispreferred to use a saturated solution preferably maintained at atemperature of not less than 40C, generally between 40 and 100C. Thecalculated amount of silver iodide is gradually added as a solid to thesolution with constant stirring. Although an apparent initial reactionseems to occur in that the mixture rapidly thickens, it has been foundthat by continued addition of the silver iodide accompanied by stirring,this thickened solution thins out to a paste as formation of the desiredcompound occurs.

While the process may be carried out at any suitable temperature, forexample, from room temperature to the boiling point of the saturatedalkali metal iodide solution, a preferred reaction temperature is above40C, generally between 40 and 100C, and particularly between 40 and 70C.After reaction occurs, the paste solution or amber-colored solution isevaporated to dryness. The solid which remains is the ionicallyconductive material MAg,,l substantially free from the nonconductivematerial M Agl The solid is generally further dried by heating it at atemperature of about 50-450C for 0.5 to 5 hours, preferably between 50and 100C. By suitable control of the conditions of precipitation andevaporation, thin films or crystals of the ionically conductivecompositions may be produced in a relatively simple manner, sinceremoval of the water by evaporation leaves the compound behind.

The following examples serve to illustrate the invention, but are in noway considered limiting thereto:

EXAMPLE 1 A saturated solution of Rbl in water is prepared by dissolving0.44 g. Rbl in a minimal quantity of water at room temperature. Silveriodide is gradually added to the solution, and a clear orange-tan liquidresults. On further addition of silver iodide, precipitation occurs.Continued addition of silver iodide results in the solution becomingclear. Continued further addition of silver iodide to a total weight of1.90 g. results in formation of a precipitate. The solution containingthe precipitate is warmed to a temperature above 40C, and a clearorange-tan solution is obtained. Upon evaporation of this solution, awhite precipitate remains as residue. This is further dried on a hotplate. X-ray diffraction of the precipitate confirmed that it was RbAgland that Rb Agl was absent. Additional confirmation of the formation ofthe desired compound was obtained by observing the known solid statetransformation temperature of 207K using differential scanningcolorimetry.

EXAMPLE 2 Following the general procedure shown in Example 1, KAg,I isprepared by reacting 0.33 g. Kl dissolved in 0.3 g. water with 1.88 g.Agl.

EXAMPLE 3 Following the general procedure shown in Example 1, NH,Ag,I isprepared by reacting 0.29 g. NHJ dissolved in 0.3 g. water with 1.88 g.Agl.

It will of course be realized that various modifications can be made inthe design and operation of the present invention without departing fromthe spirit thereof. Thus, while the principle, preferred constructionand mode of operation of the invention .have been explained and what isnow considered to represent its best embodiment has been illustrated anddescribed, it should be understood that within the scope of the appendedclaims the invention may be practiced otherwise than as specificallyillustrated and described.

I claim:

1. A process for preparing an ionically conductive composition of matterof the general formula MAg 1 where M IS Rb, K, NH Cs, or combinationsthereof, s

being present only in combination and as a minor constituent of M,comprising preparing a saturated aqueous solution of MI, intimatelyadmixing four molar equivalents of Agl with said solution to effectreaction between the MI and Agl, and removing the water to recover MAg las the synthesized ionically conductive composition.

2. The process according to claim 1 wherein M is potassium.

3. The process according to claim 1 wherein M is rubidium.

4. The process according to claim 1 wherein M is ammonium.

5. The process according to claim 1 wherein the temperature of thereaction is between about 40C and the boiling point of the solution.

6. The process according to claim 1 wherein the saturated solution of MIis prepared at room temperature, the silver iodide is gradually added tothe saturated solution with constant stirring while raising the solutiontemperature to between about 40C and the boiling point of the solution,the solution is cooled following reaction to effect precipitation of MAgl and the water is removed so that the so-prepared MAg,l,, is therebyrecovered.

7. The process according to claim 6 wherein M is potassium.

8. The process according to claim 6 wherein M is rubidium.

9. The process according to claim 6 wherein M is ammonium.

1. A process for preparing an ionically conductive composition of matterof the general formula MAg4I5 where M is Rb, K, NH4, Cs, or combinationsthereof, Cs being present only in combination and as a minor constituentof M, comprising preparing a saturated aqueous solution of MI,intimately admixing four molar equivalents of AgI with said solution toeffect reaction between the MI and AgI, and removing the water torecover MAg4I5 as the synthesized ionically conductive composition. 2.The process according to claim 1 wherein M is potassium.
 3. The processaccording to claim 1 wherein M is rubidium.
 4. The process according toclaim 1 wherein M is ammonium.
 5. The process according to claim 1wherein the temperature of the reaction is between about 40* C and theboiling point of the solution.
 6. The process according to claim 1wherein the saturated solution of MI is prepared at room temperature,the silver iodide is gradually added to the saturated solution withconstant stirring while raising the solution temperature to betweenabout 40* C and the boiling point of the solution, the solution iscooled following reaction to effect precipitation of MAg4I5, and thewater is removed so that the so-prepared MAg4I5 is thereby recovered. 7.The process according to claim 6 wherein M is potassium.
 8. The processaccording to claim 6 wherein M is rubidium.